1. Field of the Invention
The present invention relates to an optical recording device using an organic film and, more particularly, to an optical recording device which utilizes a charge transfer phenomena between donor molecules (electron donative molecules) and acceptor molecules (electron acceptive molecules) and an information writing method thereof.
2. Description of the Prior Art
Recently, computer technology has been developed to a remarkable extent, with widespread application thereof. Along with this trend, it becomes increasingly important to efficiently accumulate a large amount of information in a small storage space, and to process it at a high speed. To meet this demand, various types of information recording devices have been developed recently. Above all, information recording devices using an optical disk are attracting a great deal of attention because of their high performance, high bit density and low cost, and are being developed widely. For example, studies on various types of optical recording media using inorganic chalcogenides or organic coloring materials are increasing these days.
Meanwhile, apart from the recording system of this type, intensive studies are now being made on a data storage system with the bit density of 10.sup.10 bit/cm.sup.2 or higher such as a photochemical hole-burning recording system, wherein multiple recording is performed by making use of the discrepancy of fine structure of excited vibronic states of an organic molecule for the purpose of higher density data storage.
The conventional recording system, however, has various drawbacks. First, most of the conventional optical recording systems use "heat-mode" recording media, whether organic or inorganic. For example, recording is performed by utilizing the fact that a recording medium melts or evaporates due to thermal energy caused by absorbed photons. Information is read by detecting a difference in optical reflection or absorption intensity at a recording site. With this system, an optical energy density must be very high to accomplish a recording. When a signal having a predetermined detection level is to be obtained by using an inorganic chalcogenide series recording medium, e.g., Te and its analogues, the energy density of a recording light must be at least 3 to 100 mJ/cm.sup.2. As a result, a sufficiently high recording speed cannot be obtained at a finate power of light source such as semiconductor laser diode.
Another recording system, different from the heat mode system, is based on photon-mode recording such as a photochromic phenomenon utilizing a change cause by absorbed photons in the molecular structure of an organic molecule. In this system, since a molecular structure and conformation are significantly changed, the characteristics of the recording site such as isomerization and bond cleavage are unstable and tend to change over time. Still another new optical data storage system utilizes photochemical hole-burning. Since this system utilizes tautomerism of proton transfer in an organic molecule, such as porphyrin and phthalocyanine (in rigid matrices), both the conversion efficiency and speed are low and small, and its noise level cannot be decreased unless at low temperature less than 100.degree. K. For these reasons, a high-density recording system of 10.sup.6 bits/cm.sup.2 or higher is difficult to provide, despite great potential demand for it.
A critical defect of the conventional optical recording device is that, once information is written, it cannot be erased to allow new information to be rewritten. Optical magnetic disks as erasable recording media have been developed in order to eliminate this defect. Since this system reads a magnetizing direction due to a magnetic Kerr effect as a recorded signal, however, a high S/N ratio inevitably cannot be obtained, an expensive optical system to detect the signals with high resolution is required for reading, and an expensive rare earth elements must be used in order to obtain the efficient Kerr rotation. For these reasons, this system is limited only to a special application and thus lacks versatility.
Recent developments in understanding the structure, properties and synthesis of organic materials may realize a new functional element that uses such a well-characterized organic material. In particular, since a technique has advanced for ordering organic molecules in condensed phases and stacking their layers with the thickness of 10 .ANG. or lower, which is represented by the Langmuir-Blodgett process (to be referred to as the LB process hereinafter), a monolayer of organic material can be easily formed and built-up as a controlled layer structure. This may allow development of a new functional element by making use of the optical and electrical properties of organic molecules in controlled state. In fact, Roberts et al., at Durhram University in the U.K., report a semiconductor device, such as a MIS type FET and a MIS type light-emitting element, which uses an organic thin film of the thickness below 100 .ANG., formed by the LB process, as an insulating film on semiconductors. Although an organic thin film obtained by the LB process is expected to serve as an optical recording medium because of its good optical response characteristics of organic molecules and the characterized film structures, it has not yet been put into practical use.
As described above, various types of optical recording devices and optical recording systems that are conventionally proposed do not sufficiently utilize the characteristic features of super high recording density as optical recording in improving their writing speed, contrast, stable storage performance of recording conditions, and so on.